The invention relates to the generation of frequency tunable electromagnetic signals using optical lasers, in particular but not exclusively to the production of microwave signals.
U.S. Pat. No. 4,907,234 discloses a laser diode construction for microwave production. The device involves two slave lasers injection locked to a master laser. The master laser is subject to microwave modulation to produce sidebands to the central frequency. The slave lasers are injection locked to different sidebands of the master laser. The outputs of the slave lasers are then heterodyned to produce a microwave output. The device requires an external source of microwaves.
U.S. Pat. No. 4,930,133 discloses a laser diode construction for microwave production. The outputs of two lasers are fed into an external resonant cavity and then fed back into the lasers to effect injection locking to different resonant modes of the cavity. The outputs of the two lasers are then heterodyned to produce a microwave output. The device requires the presence of an external resonator.
In xe2x80x9cLinewidth Narrowed Vertical-Cavity Surface-Emitting Lasers for Millimeter-Wave Generation by Optical Heterodyningxe2x80x9d P. Dowd et. al. Journal of selected topics in Quantum Electronics, Vol. 3, No. 2, April 1997, a device is described using two lasers and heterodyning their outputs. The lasers involve the use of an external fiber-loop mirror as an external cavity for the lasers. Such a fiber-loop mirror is expensive.
The invention solves the problem of providing a microwave generator which does not require an external reference signal and does require expensive components.
An electromagnetic signal is produced by heterodyning the outputs of two or more optical lasers. A beat frequency signal produced by the heterodyning is fed back into the inputs of the lasers to produce sidebands in the optical output. The each laser is then optically injection-locked to a sideband of the other laser so that the lasers differ in frequency by an amount equal to the beat frequency. The beat frequency may be varied by varying the input bias to one or more of the lasers.